The nucleoids of Escherichia coli cells undergo three major modifications after infection with T-even bacteriophages: (1) "unfolding" from a 2000-3000S negatively supercoiled DNA structure maintained by an "RNA core" to a 155S relaxed circular double-helix of DNA, (2) "nuclear disruption," during which the host DNA moves from its largely central location in the cell into tight juxtaposition with the cell membrane, and (3) degradation of the host DNA to nucleotides which are subsequently incorporated into progeny viral DNA. We have isolated mutants of phage T4 which fail to induce each of these modifications, and have constructed multiple mutant strains deficient in all or any combination of these modifications. These strains are being studied genetically and biochemically in an attempt to elucidate the mechanism responsible for and biological significance of each of these modifications.